1. Field of the Invention
The present invention relates to a circuit for driving a light emitting element such as a semiconductor laser and light emitting diode used in, for example, the field of optical communications and the like, and particularly to a light emitting element driving circuit capable of driving such an element at a low voltage and at a high speed.
2. Description of the Related Art
In a light transmitting part such as in an optical transmission apparatus adopting a light emitting element, there is provided a light emitting element driving circuit for converting data of electrical signals to be transmitted into optical signals. Also, in an information-processing equipment such as an optical disk apparatus and a laser printer having light emitting elements, there is provided a light emitting element driving circuit similarly to the optical transmission apparatus.
FIG. 22 shows an exemplary general constitution of a light emitting element driving circuit such as for high-speed optical communications.
In the circuitry shown in FIG. 22, there are provided an electric current source 1, a light emitting element 2 and a differential type electric current switch 3, between a terminal applied with a supply voltage VDD and a ground terminal, in which a path of a constant current generated by the electric current source 1 is switchedly controlled by the current switch 3 in accordance with a transmission data signal DATA and a reversed transmission data signal XDATA so as to drive the light emitting element 2.
As the current source 1 to be used here, it is typical to adopt such as a current mirror circuit which utilizes a plurality of transistors.
Meanwhile, supply voltages for recent digital CMOS-LSI""s used in various information/communications related equipments have been lowered corresponding to the demand of decreased power consumption and high-precision processing. As such, supply voltages have been presently lowered to the order of 2.5 V to 1 V. Such advancement of lower supply voltages of various equipments leads to supply voltages lower than those required for driving light emitting elements, thereby requiring a countermeasure such as to boost supply voltages.
Conventional light emitting element driving circuits where boosting of supply voltages is performed include one disclosed in Japanese Unexamined Utility Model Publication No. 6-73713. This conventional driving circuit is constituted to drive a light emitting element such as for a camera at a voltage higher than a supply voltage, by charging and discharging a capacitor. Further, the switching element for driving the light emitting element is commonized with that for a known bridge circuit for driving a motor so as to electrically separate the driving circuit of the light emitting element from other circuits constituting the system of camera, to thereby contemplate downsizing the system of camera as a whole and improving reliability.
Although the aforementioned conventional light emitting element driving circuit may be sufficient to cope with a lowered supply voltage, such a circuit is problematic as one for optical communications where a stable high speed modulation is required.
Namely, in the light emitting element driving circuit for high-speed optical communications as mentioned above, the current source 1 is provided to supply a stable and constant driving current to the light emitting element 2, and there is frequently used a current mirror circuit adopting a plurality of transistors as the current source 1. Since the transistors in such a current mirror circuit have parasitic capacitances, the direct modulation of the light emitting element 2 by using the current mirror circuit as the current source 1 leads to a transiently unstable value of a driving current for the light emitting element 2 during the flow of charging and discharging currents into and from the aforementioned parasitic capacitances, resulting in difficulty of the high speed modulation. To stably conduct the high speed modulation, it is necessary to control the operation of the current source 1 corresponding to the driving state of the light emitting element 2 simultaneously with the controlling of charging and discharging of the capacitor.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a light emitting element driving circuit for optical communications, capable of coping with a lowered supply voltage and of realizing a stable high speed modulation.
To achieve the above object, the present invention provides a light emitting element driving circuit for driving a light emitting element by a constant electric current generated by an electric current source, comprising: a first switching part for switching a portion of a path for an electric current flowing into the electric current source, to one of a light emission path including the light emitting element and a light extinction path excluding the light emitting element, in accordance with a data signal for determining whether the light emitting element is to be driven for light emission; a supply voltage boosting part for boosting a supply voltage based on a charge accumulated by the supply of the supply voltage; a second switching part for switching another portion of the path for an electric current flowing into the electric current source, to one of a charging path for charging the supply voltage boosting part and a boosting path for applying the supply voltage boosted by the supply voltage boosting part to the light emitting element, in accordance with a control signal corresponding to the data signal.
According to such a constitution, the first switching part and the second switching part operate in accordance with the data signal and the control signal, respectively, to thereby switch the paths for an electric current flowing into the electric current source corresponding to the driving state (light emission or light extinction) of the light emitting element, so that the supply voltage boosted by the supply voltage boosting part is applied to the light emitting element to thereby drive the same. At this time, the electric current paths to be switched pass through the electric current source irrespectively of the driving state of the light emitting element, to thereby continuously keep the electric current source in an operating state. This enables a stable high speed modulation of the light emitting element.
Concerning the light emitting element driving circuit, constitution may be such that the supply voltage boosting part includes a capacitor having a first terminal and a second terminal; and when the switching to the charging path is conducted, the second switching part applies the supply voltage to the first terminal of the capacitor and connects the second terminal of the capacitor to a path leading to the electric current source, and when the switching to the boosting path is conducted, the second switching part applies the supply voltage to the second terminal of the capacitor and connects the first terminal of the capacitor to the light emitting element.
According to such a constitution, the switching between charging and discharging (boosting) is conducted by the second switching part in accordance with the control signal, and the supply voltage is boosted by the charge accumulated in the capacitor to be supplied to the light emitting element.
As a concrete constitution of the light emitting element driving circuit, it is possible that the control signal is common to the data signal; when the data signal is at a logic to bring the light emitting element into a light extinction state, the switching operations of the first switching part and the second switching part are to form a path of an electric current flowing through: the first terminal of the capacitor applied with the supply voltage; the second terminal of the capacitor; and the electric current source; in this order, to thereby charge the capacitor; and when the data signal is at a logic to bring the light emitting element into a light emission state, the switching operations of the first switching part and the second switching part are to form a path of an electric current flowing through the second terminal of the capacitor applied with the supply voltage, the first terminal of the capacitor, the light emitting element and the electric current source, in this order, to thereby drive the light emitting element by the boosted supply voltage.
According to such a constitution, the first and second switching parts are switched in accordance with the data signal, respectively, such that the capacitor is charged when the light emitting element is to be brought into a light extinction state and such that the voltage boosted by the capacitor is applied to the light emitting element when the light emitting element is to be brought into a light emission state.
As another concrete constitution of the light emitting element driving circuit, it is possible that the control signal is a cell signal indicative of whether the data signal is at an intracellular timing including logics for bringing the light emitting element into a light emission state or at an extracellular timing; when the cell signal is at a logic indicative of an extracellular timing, the switching operations of the first switching part and the second switching part are to form a path of an electric current flowing through the first terminal of the capacitor applied with the supply voltage, the second terminal of the capacitor and the electric current source, in this order, to thereby charge the capacitor; and when the cell signal is at a logic indicative of an intracellular timing and the data signal is at a logic to bring the light emitting element into a light emission state, the switching operations of the first switching part and the second switching part are to form a path of an electric current flowing through the second terminal of the capacitor applied with the supply voltage, the first terminal of the capacitor, the light emitting element and the electric current source, in this order, to thereby drive the light emitting element by the boosted supply voltage.
According to such a constitution, the first switching part is switched in accordance with the data signal and the second switching part is switched in accordance with the cell signal, so that the capacitor is charged at the extracellular timing where the light emitting element is not brought into a light emission state. Further, when the light emitting element is to emit light at the intracellular timing, the voltage boosted by the capacitor is applied to the light emitting element.
Further, it is preferable for the light emitting element driving circuit that when the cell signal is at a logic indicative of an intracellular timing and the data signal is at a logic to bring the light emitting element into a light extinction state, the switching operations of the first switching part and the second switching part form a path of an electric current excluding the capacitor and the light emitting element.
According to such a constitution, there can be formed the electric current path to avoid useless consumption of the charge accumulated in the capacitor, when the light emitting element is to extinguish light at the intracellular timing.
Further, concerning the aforementioned light emitting element driving circuit, this circuit may comprise a data detecting part for detecting whether the data signal within a finite length includes a logic to bring the light emitting element into a light emission state and the detection result of the data detecting part may be used as the cell signal.
According to such a constitution, it is detected, at the data detecting part, whether a logic to bring the light emitting element into a light emission state exists in the data signal within the finite length, and a signal corresponding to the cell signal is generated within the light emitting element driving circuit.
In addition, concerning the aforementioned light emitting element driving circuit, it is possible that: the control signal comprises a signal based on a clock signal synchronized with the data signal and the cell signal; the light emitting element driving circuit further comprises a rectifying part for rectifying the supply voltage boosted by the capacitor and for applying the thus boosted supply voltage to the light emitting element; when the cell signal is at a logic indicative of an extracellular timing and the signal based on the clock signal is at one of two indicatable logics, the switching operations of the first switching part and the second switching part are to form a path of an electric current flowing through the first terminal of the capacitor applied with the supply voltage, the second terminal of the capacitor and the electric current source, in this order, to thereby charge the capacitor; when the cell signal is at a logic indicative of an extracellular timing and the signal based on the clock signal is at the other of two indicatable logics, the switching operations of the first switching part and the second switching part are to form a path of an electric current flowing through the second terminal of the capacitor applied with the supply voltage, the first terminal of the capacitor and the rectifying part, in this order; and when the cell signal is at a logic indicative of an intracellular timing and the data signal is at a logic to bring the light emitting element into a light emission state, the switching operations of the first switching part and the second switching part are to form a path of an electric current flowing through the rectifying part, the light emitting element and the electric current source in this order, to thereby drive the light emitting element by the voltage rectified by the rectifying part.
According to such a constitution, the charging and discharging of the capacitor at the extracellular timing is conducted in accordance with the signal (clock signal, or a signal obtained by dividing the clock signal) based on the clock signal; and when the light emitting element is to be brought into a light emission state at the intracellular timing, the voltage boosted by the capacitor and rectified by the rectifying part is applied to the light emitting element.
Further, the aforementioned light emitting element driving circuit may further comprise: a sequence controlling part for controlling the operation of the second switching part such that the supply voltage to the supply voltage boosting part is momentarily interrupted when switching between the charging path and the boosting path at the second switching part.
According to such a constitution, the operation of the second switching part is sequence controlled by the sequence controlling part when switching between the charging path and boosting path, so as to prevent the supply voltage boosting part from entering a short-circuited condition. This eliminates unnecessary discharge at the supply voltage boosting part, and enables avoidance of deterioration of the voltage transforming efficiency.
Other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.